Ellen S. Richards Monographs No. 2 
Published by Vassar College 


The Discovery 
of 


Radium 


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Address by 
Madame M. Curie 


at Vassar College 
May 14, 1921 


PREFATORY NOTE 


N HER recent visit to America, Madame Curie 

conferred a special honor upon Vassar College by 
delivering in the chapel on the evening of May four- 
teenth the only extended address which she made in 
this country. In a simple, straightforward way she 
told the story of her great achievement. One realized 
how, closely environed by all the great realities of human 
experience, in the face of tremendous difficulties and 
with limited resources, she had pursued undaunted her 
search for truth. 


The discovery of radium gave Madame Curie im- 
mediate distinction among scientists on account of the 
extremely significant contribution she thereby made to 
the great ultimate problem of physical science, the 
constitution of matter. The striking properties pos- 
sessed by radium gave to its discovery a world-wide 
interest, all the more intense because of the hope which 
was inspired by the possible healing saan of the 
radiations from this new element. 


That hope is being realized in large measure. It is 
therefore fitting that this address should have been 
given by Madame Curie at Vassar and that it should 
now be circulated among the members of the college 
under the foundation in memory of Ellen S. Richards, 
who devoted her life to the public health. 


Edna Carter 


Chairman of the 
Department of Physics. 


THE DISCOVERY OF RADIUM 


I could tell you many things about radium and radioactivity 
and it would take a long time. But as we can not do that, I 
shall only give you a short account of my early work about 
radium. Radium is no more a baby, it is more than twenty 
years old, but the conditions of the discovery were somewhat 
peculiar, and so it is always of interest to remember them and 
to explain them. 


We must go back to the year 1897. Professor Curie and | 
worked at that time in the laboratory of the school of Physics 
and Chemistry where Professor Curie held his lectures. I was 
engaged in some work on uranium rays which had been dis- 
covered two years before by Professor Becquerel. I shall tell 
you how these uranium rays may be detected. If you take a 
photographic plate and wrap it in black paper and then on this 
plate, protected from ordinary light, put some uranium salt 
and leave it a day, and the next day the plate is developed, 
you notice on the plate a black spot at the place where the 
uranium salt was. This spot has been made by special rays 
which are given out by the uranium and are able to make an 
impression on the plate in the same way as ordinary light. 
You can also test those rays in another way, by placing them 
on an electroscope. You know what an electroscope is. If 
you charge it, you can keep it charged several hours and more, 
unless uranium salts are placed near to it. But if this is the 
case the electroscope loses its charge and the gold or aluminum 
leaf falls gradually in a progressive way. The speed with which 
the leaf moves may be used as a measure of the intensity of 
the rays; the greater the speed, the greater the intensity. 


I spent some time in studying the way of making good 
measurements of the uranium rays, and then I wanted to know 
if there were other elements, giving out rays of the same kind. 
So I took up a work about all known elements, and their com- 
pounds and found that uranium compounds are active and also 


all thorium compounds, but other elements were not found 
active, nor were their compounds. As for the uranium and 
thorium compounds, I found that they were active in propor- 
tion to their uranium or thorium content. The more uranium 
or thorium, the greater the activity, the activity being an atomic 
property of the elements, uranium and thorium. 


Then I took up measurements of minerals and I found that 
several of those which contain uranium or thorium or both 
were active. But then the activity was not what I could 
expect, it was greater than for uranium or thorium compounds 
like the oxides which are almost entirely composed of these 
elements. Then I thought that there should be in the minerals 
some unknown element having a much greater radioactivity 
than uranium or thorium. And I wanted to find and to sepa- 
rate that element, and I settled to that work with Professor 
Curie. We thought it would be done in several weeks or 
months, but it was not so. It took many years of hard work 
to finish that task. There was not one new element, there 
were several of them. But the most important is radium 
which could be separated in a pure state. 


All the tests for the separation were done by the method 
of electrical measurements with some kind of electroscope. 
We just had to make chemical separations and to examine all 
products obtained with respect to their activity. The product 
which retained the radioactivity was considered as that one 
which had kept the new element; and, as the radioactivity was 
more strong in some products, we knew that we had succeeded 
in concentrating the new element. The radioactivity was 
used in the same way as a spectroscopical test. 


The difficulty was that there is not much radium in a mineral; 
this we did not know at the beginning. But we now know 
that there is not even one part of radium in a million parts of 
good ore. And too, to get a small quantity of pure radium 
salt, one is obliged to work up a huge quantity of ore. And 
that was very hard in a laboratory. 


_ We had not even a good laboratory at that time. We 
worked in a hangar where there were no improvements, no 
good chemical arrangements. We had no help, no money. 
And because of that the work could not go on as it would have 
done under better conditions. I did myself the numerous 
crystalizations which were wanted to get the radium salt 
separated from the barium salt with which it is obtained out 
of the ore. And in 1902 I finally succeeded in getting pure 
radium chloride and determining the atomic weight of the 
new element radium, which is 226 while that of barium is only 


137. 


Later I could also separate the metal radium, but that was 
a very difficult work; and, as it is not necessary for the use of 
radium to have it in this state, it is not generally prepared 
that way. 


Now, the special interest of radium is in the intensity of its 
rays which is several million times greater than the uranium 
rays. And the effects of the rays make the radium so important. 
If we take a practical point of view, then the most important 
property of the rays is the production of physiological effects 
on the cells of the human organism. These effects may be 
used for the cure of several diseases. Good results have been 
obtained in many cases. What is considered particularly 
important is the treatment of cancer. The medical utilization 
of radium makes it necessary to get that element in sufficient 
quantities. And so a factory of radium was started to begin 
with in France, and later in America where a big quantity of 
ore named carnotite is available. America does produce many 
grams of radium every year but the price is still very high 
because the quantity of radium contained in the ore is so small. 
The radium is more than a hundred thousand times dearer 
than gold. . 


But we must not forget that when radium was discovered 
no one knew that it would prove useful in hospitals. The 
work was one of pure science. And this is a proof that scientific 


work must not be considered from the point of view of the 
direct usefulness of it. It must be done for itself, for the 
beauty of science, and then there is always the chance that 
a scientific discovery may become like the radium a benefit 
for humanity. 


But science is not rich, it does not dispose of important 
means, it does not generally meet recognition before the ma- 
terial usefulness of it has been proved. The factories produce 
many grams of radium every year, but the laboratories have 
very small quantities. It is the same for my laboratory and 
I am very grateful to the American women who wish me to 
have more of radium and give me the opportunity of doing 
more work with it. 


The scientific history of radium is beautiful. The properties 
of the rays have been studied very closely. We know that 
particles are expelled from radium with a very great velocity 
near to that of the light. We know that the atoms of radium 
are destroyed by expulsion of these particles, some of which 
are atoms of helium. And in that way it has been proved 
that the radioactive elements are constantly disintegrating 
~and that they produce at the end ordinary elements, principally 
helium and lead. That is, as you see, a theory of transforma- 
tion of atoms which are not stable, as was believed before, 
but may undergo spontaneous changes. 


Radium is not alone in having these properties. Many 
having other radioelements are known already, the polonium, 
the mesothorium, the radiothorium, the actinium. We know 
also radioactive gases, named emanations. There is a great 
variety of substances and effects in radioactivity.. There is 
always a vast field left to experimentation and | hope that we 
may have some beautiful progress in the following years. It 
is my earnest desire that some of you should carry on this 
scientific work and keep for your ambition the determination 
to make a permanent contribution to science. 


M. Curie. 


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